Due to fly on BA292 tonight from IAD-LHR. Scheduled departure of 22:20 but it's not leaving now until 00:52 (the inbound flight hasn't even left LHR yet).

Needless to say, I don't want to spend hours at Dulles tonight especially when I know the flight will be late.

Does anyone have advice as to what time I should get to IAD? I've checked in on-line but want to make sure the checkin desks are going to be open should I get there after the original departure time but before the actual new departure time.